1. Field of the Invention
The present invention relates to optical sensor systems and more specifically to battery powered fiber optic sensor system.
2. Description of the Prior Art
Optical sensor systems, in which a sensed condition such as temperature, pressure, acceleration or sound is converted to an optical signal for transmission to a remote utilization apparatus are well known in the art.
These systems frequently employ a transducer such as a piezoelectric element, to convert a sensed condition, such as a pressure variation to an electrical signal that actuates a variable light source. The output of the light source is then coupled into a fiber optic element which transmits the light energy to a remote measuring position.
Such fiber optic systems frequently employ either single mode interferometric techniques or multimode intensity modulated techniques. In the latter case, light is transmitted to and from the sensor location and the sensor acts merely to modulate the returned light signal in accordance with the condition being monitored.
In terms of reliability and cost, there is frequently great value attached to the use of such electrically passive fiber optic sensors.
In many applications, however, complete electrical sensor passivity is less important than overall power minimization with respect to the entire fiber optic sensor system. For such systems, transmitting optical power to the sensor for optical modulation and for ultimate conversion of the modulated signal to electrical power for running electronic components is less efficient than incorporating a battery into the sensor circuit.
In such prior art devices, the fiber optic sensors typically operate at relatively high optical power levels so as to minimize optical shot noise with respect to the smallest possible change in optical throughput caused by the sensor. The shot noise may be expressed as: ##EQU1## where .DELTA.I/I is the minimum fractional optical signal change that is just detectable, I is the optical power level reaching the light detector in Watts, and B is the detection bandwidth in Hertz.
Thus, adopting the convention that an exponentional term A10.sup.-M may be written AE-M, and assuming that I=100 .mu.W, and B=1 Hz, one can just detect a change in signal of .DELTA.I=8E-10.times.1E-2=8E-12 Watts.
Assuming the sensor is mechanically constructed to maximize the optical throughput change for a given change in monitored condition, further increases in sensitivity (in view of the shot noise limit) can be achieved only by increasing the optical signal power reaching the detector.
Thus in the prior art devices, there is an apparent conflict between the goal of maximizing sensitivity and minimizing the electrical power supplied to drive the light source. The resolution of this conflict is an object of the present invention.